This invention relates generally to test circuitry and more particularly to test circuitry adapted to measure the compliance voltage range of current output devices such as current output digital-to-analog converters (DAC's).
As is known in the art, it is frequently desired to determine the maximum range of output terminal voltage for which a current output device, such as a current output DAC, will provide a specified current output characteristic. This voltage range is typically referred to as the compliance voltage range of the device. The compliance voltage range may alternatively be viewed as the voltage range at the output of a current source device over which the current source will maintain a given accuracy. One such current source device is a current output DAC which produces a current output having a magnitude typically proportional to an n-bit digital word fed to the device. The DAC thus may be considered as a digitally controlled current source, the level of the current produced by such source being proportional to a digital word fed to the device.
As is also known in the art, while the output of the DAC is a current, this output is typically converted into a proportional voltage. Such conversion may, for example, result by connecting a transistor to the output of the DAC, in which case an output voltage is produced across the resistor which is proportional to the current produced by the DAC, or in another example, the output of the DAC is fed to an operational amplifier having a feedback resistor which produces an output voltage proportional to the current produced at the output of the DAC. In either example, however, because of the internal impedance of the DAC, the DAC cannot be considered as an "ideal" current source and hence the output current will change to some degree from that defined by the digital word fed to it because of the current through the internal impedance resulting from the voltage produced at the output terminal of the DAC. This change or variation in the desired output current as a function of output voltage is the compliance voltage range of the DAC and such must be measured to advise the user of the DAC so that such user is able to determine whether the DAC with such specified compliance is suitable for the user's particular application.
One technique used to measure compliance voltage range is to connect a voltage source with a current measuring capability to the output of the DAC. The DAC is typically set to full scale (i.e. maximum) current output and the voltage of the voltage source is varied while the change in output current is measured. While such technique has been used successfully in some applications, the technique is generally inadequate using automatic test equipment with 10 bit DAC devices requiring a relatively higher degree of measuring accuracy, since accuracy to one-half the least significant bit must be measured, typically 2 microamps, in the presence of typically 0.2 microamps of noise. Still further accuracy is required where so-called "functional compliance" (i.e. differential non-linearity or relative accuracy) must be measured with finite, non-zero voltages produced at the output of the DAC.